This invention relates in general to aligning objects and more particularly to superimposing alignment marks, as well as other patterns, on two or more plates. The term plate includes masks, such as photomasks and X-ray lithography masks, reticles, wafers, substrates and other solid, generally flat, objects.
In many microlithographic techniques such as photolithography, and X-ray lithography (see soft X-ray lithographic apparatus and process, Smith et al., U.S. Pat. No. 3,743,842, July 3, 1973) it is frequently desirable to expose several separate masks on the same substrate, and the patterns produced by these several masks (the "primary patterns") must be precisely superimposed. One method of achieving this is to place on each of the several masks of a set, alignment marks having spatial relationships to the primary patterns that are exactly the same on each mask. Thereafter, superposing of the mask's alignment marks relative to corresponding alignment marks on the substrate ensures superposition of primary patterns on the substrate. In many cases, extensive processing of a substrate, such as doping, high temperature baking, and other steps, occurs between each exposure, and it is important that the substrate alignment marks neither be destroyed nor lead to contamination or alteration of the substrate or to processing problems. If the minimum feature size of patterns to be exposed on a substrate is of the order of 2.5 micrometer (.mu.m), superposition precision of the order of 0.5 .mu. m is often adequate. In such cases, ordinary optical imaging techniques can be used. Alignment is achieved by superimposing two or more mask alignment marks (typically in the form of an L or a T) over similar alignment marks on a substrate. Both rotational and translational alignment is achieved by viewing the two sets of alignment marks through a microscope. With this technique it is difficult to simultaneously view with high resolution an alignment mark on the mask and one on the substrate when there is a finite separation or gap between said alignment marks. This is because the depth of focus of high resolution lenses is severely limited. Moreover, the technique is inadequate for superposition precisions of 100 nanometers (nm) and below, which will be required by the new microlithographic techniques of X-ray lithography, conformable photomask lithography and far UV lithography (see B. J. Lin, Deep UV Conformable-Contact Photolithography for Bubble Circuits, IBM Journal, p. 213, May 1976). These techniques are capable of exposing patterns with minimum feature sizes below 1 .mu.m and thus require correspondingly improved superposition precision, superior to that provided by the conventional optical imaging techniques referred to above.
A soft X-ray mask alignment system (see "Soft X-Ray Mark Alignment System", Smith et al., U.S. Pat. No. 3,742,229) has been patented that is based on the detection of soft X-rays transmitted through matching alignment marks on an X-ray mask and a substrate. A disadvantage of this system is the requirement that the substrate be transparent to soft X-rays in the region behind the alignment marks on the substrate so that X-rays can pass through the substrate to a soft X-ray sensor. This requires either a thinning of the substrate in these regions or, in some cases, the drilling of holes entirely through the substrate. Another soft X-ray mask alignment scheme (see "Soft X-ray Mask Alignment System", H. I. Smith, U.S. Pat. No. 3,984,860) has also been patented that is based on the detection of soft X-rays or other radiation emitted by fluorescent material on the substrate. These X-rays are detected by means of a detector located away from the substrate in the direction of the mask. Disadvantages of this system include poor signal-to-noise ratio and possible contamination of the substrate by the material of the substrate alignment mark.
M. C. King and D. H. Berry proposed an alignment scheme that is superior in performance to conventional optical imaging techniques (see M. C. King, U.S. Pat. No. 3,690,881). The alignment marks on the mask and the substrate consist of concentric circles, those on the mask having slightly different pitch than those on the substrate. Superposition is achieved by viewing the moire pattern formed when the mask alignment marks are placed above the substrate alignment marks. At misalignment, a cusp-like moire fringe pattern is formed. At perfect superposition, the moire pattern is a set of concentric fringes centered over the image of the alignment marks. King and Berry claimed a superposition precision of 200 nm (M. C. King and D. H. Berry, Applied Optics, Vol. 11, page 2455 et seq. 1972). A difficulty with this scheme is that maximum moire fringe contrast is achieved when imaging is done in an optical system of low numerical aperture, a requirement that is inconsistent with the need to image the gratings themselves. Also, the scheme is not compatible with large mask-to-substrate separations. An additional problem is that the concentric circle alignment marks required by this scheme are difficult or costly to generate by conventional pattern generation means. Also, because of the complexity of the moire fringe pattern, it is difficult to implement an automatic mask alignment system based on this scheme.
It is therefore an object of this invention to provide an alignment system for superimposing alignment marks on two or more plates (defined here to include photomasks, X-ray lithography masks, other masks, reticles, wafers, substrates and other solid, generally flat objects) and capable of submicrometer superposition precision.
It is a further object of this invention to provide an alignment system that will function properly when the separation distance between two or more of the plates to be superimposed exceeds several micrometers.
It is a further object of this invention to provide a means for superimposing alignment marks on two or more plates when one plate may be opaque to the radiation used for alignment.
It is a further object of this invention to provide for the superposition of the alignment marks of each of a set of plates relative to the alignment marks on a single plate.
It is a further object of this invention to provide a means of determining the separation distance between facing alignment marks.
It is a further object of this invention to achieve one or more of the preceding objects using alignment marks on a plate that are non-contaminating to said plate.
It is a further object of this invention to achieve one or more of the preceding objects using alignment marks that have a geometrically simple pattern that can be readily generated by conventional pattern generation means.
It is a further object of this invention to achieve one or more of the preceding objects in an automated alignment system.